1. Field of the Invention
The present invention concerns multiple-beam microwave tubes with longitudinal interaction, such as multiple-beam klystrons. In particular, it concerns multiple-beam klystrons with a coaxial output.
2. Description of the Prior Art
A multiple-beam klystron has N parallel longitudinal electron beams produced by one or more electron guns. The fact of splitting of a beam into several elementary beams has the advantage of reducing the space-charge effects and of giving a tube with greater efficiency. This also enables the current and power of the tube to be increased or else its operating voltage to be reduced.
Several standard single-beam klystrons can be grouped together in one and the same envelope: in this way a multiple-beam klystron is obtained. The single-beam klystrons are distributed on a ring centered on an axis. This axis is the axis of the multiple-beam klystron. The different electron beams are then parallel to this axis. This construction enables certain elements of standard single-beam klystrons to be used without any notable modification. The beams produced by each of the klystrons are then elementary beams. They go through successive cavities, each cavity being crossed by all the beams.
A standard single-beam klystron is built around an axis which is the axis of the electron beam. A microwave to be amplified is introduced into the first cavity which is on the gun side. This is the input cavity. The last cavity or output cavity is connected to an external energy-using apparatus by means of a short transmission line. The transmission line is generally positioned crosswise with respect to the axis of the tube. It receives the microwave after amplification. The electron beam is collected in a collector that is coaxial with the axis of the tube. This collector is placed downline of the output cavity. A focusing device surrounds the cavities. It prevents any divergence of the electron beam in the drift tubes and in the cavities.
In a multiple-beam klystron formed by several single-beam klystrons grouped together in one and the same envelope, the focusing device may be common to all the tubes.
The major drawback of the multiple-beam klystrons formed by the grouping together of several single-beam klystrons lies in the output of the microwave energy.
The output cavity is connected to a transmission line. The transmission line is generally lateral and may be placed transversally with respect to the axis of the tube. This construction is then dissymmetrical. The dissymmetry notably causes problems in focusing.
The focusing device cannot totally surround the output cavity connected to the lateral transmission line. The magnetic field is then reduced at this place, and this entails the risk of a disturbance in the path of the electron beams crossing this cavity. It is possible to use coils of electro-magnets cut into the transmission line, but these coils do not really make it possible to recover a proper magnetic field value. It is also possible to use a curved guide.
The dissymmetry arising out of the transmission line which is transversal also entails difficulties in the assembling of the tube. For, the assembly formed by the gun, the cavities and the collector must be slid in and fitted precisely into the focusing device. This task of manipulation is always very difficult to perform because the the mass of the assembly is are very great. The transmission line then has to be connected to the output cavity. This connection has to be very precise.
In French patent application No. 89 07784, filed on 13th June 1989, the present Applicant has already proposed a klystron type microwave tube having an output coaxial with the collector. According to one embodiment, this application describes a multiple-beam klystron built around an axis. This klystron has, chiefly, a gun producing several electron beams, successive cavities and a collector. Each cavity is crossed by all the beams. The collector located downline of the last cavity is coaxial with the axis of the tube. The last cavity is coupled to a transmission line that surrounds the collector and is coaxial with it. This transmission line is, for example, a coaxial waveguide. The coupling between the output cavity and the transmission line is achieved by at least one coupling aperture.
This construction is symmetrical at the output but, nevertheless, has other drawbacks. The collector is surrounded by the transmission line. Its diameter is limited and so are its possibilities of discharging heat. Furthermore, if the collector has to be cooled by the circulation of a liquid, the quantity of liquid that can circulate is restricted. As a consequence, this tube can work only at moderate levels of mean or peak power. By contrast, the transmission line surrounding the collector has large dimensions. If the operating frequency is high, then there is a risk that the transmission line may be oversized. Several modes may then get propagated in the transmission line, and this is not desirable.